That old devil

In the course of that wine-bar small-talk which hints at bigger things, someone you've met reveals that they're a police officer. As the flirtation thickens into dating, you notice that they have no uniform or workplace. Your soulmate explains that this is because he's an undercover cop.

When you start to live together, you're surprised that this key figure in the secret police resides in an unfurnished wreck of a council house. Again he sets your mind at rest: because his current "cover" is that of a drug-user, he must live like the scum he's infiltrating. And, encouragingly, here now is a sign that he really is the plain-clothes rozzer he claims to be. Once a week you drive him to the local nick where he goes inside briefly to "deliver paperwork". Most people faced with this anecdotal evidence would run for cover or call the cops, suspecting that there might be another possible reason why a guy living in a tip called in at the nearest police station once a week. Joanne, though, became increasingly excited by the fact that she had landed a man with a job doing as glamorous as undercover surveillance of drug barons. This is because she was suffering from Love (Tuesday, 9pm, Channel 4), the subject of a seductive documentary by Sue Bourne. Her film - following the progress of several relationships - belongs to a genre of documentaries traditionally screened on February 14. But you soon realise that this particular example has been held back until June 25 because it would have destabilised the Valentine's industry as chocolates were spat out and roses ripped from their vases.

Taking as its text the saying that love is blind, Bourne's film is an optician's test. The film's punctuating images - scudding clouds, lightning flashes, pictures darkening to negatives - build in the viewer an ominous feeling which the speakers themselves somehow ignored. The producer has slipped a little billet-doux in the advance cassettes - requesting that previewers don't give away the pay-off in the realtionships - and this request should be respected, as the film is paced and edited like a thriller, to encourage tension and doubt.

One of the three featured relationships ends miserably, one equivocally and the third deliriously. In each affair, one partner is guarding a secret but one of these is benevolent while the other two are sinister. If Sue Bourne had run a book half-way through, she would have taken a lot of money off me from my bets on the outcomes of two of the stories. Alongside Joanne, who gave up her job to live with the secret policeman she loved in his all-too-convincingly squalid squat, the other lovers telling their stories are Jon and Wendy.

The latter was in her 50s and twice divorced when she met David, who had also accumulated two decree nisis. Reversing their romantic histories, they clicked "in every way" (the approved middle-class media code for multiple and probably simultaneous orgasms) until, one day, Wendy's children played her a tape recording of a phone call in which her beloved third husband is heard hiring a business associate to run her over with his car. David on the tape stresses the importance of accuracy. He doesn't want to be left with her as a "vegetable." He can "easily find another wife".

Jon - Bourne's third exhibit - spent several years dating a woman he had met in a London pub. Though friendly, she would never let him near her home, blaming a difficult family. Jon heard rumours that her relatives associated with Popes and that her mother had royal blood, but he dismissed these as closing-time anecdotes, the way that most people would react if, say, their lover explained away a lack of house-work by claiming to be an underground narcotics agent.

If these stories were told as a newspaper feature - and, if they haven't been already, they will - there'd be a moralistic side-box in which a psychologist or agony aunt explained how the heart and the genitals derange the brain. But, taking the line that love is inexplicable, the film simply leaves the lovers to speak for themselves. Bourne's judgment of when to cut from one romance to the next is impeccable throughout, surely encouraging many viewers to be quite wrong about how things will turn out which - in a film above love - means that technique and content come together, or, as one of her interviewees would say, work in every way.